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Precision medicine is redefining what makes a cancer rare or common

A male physician reviewing rare cancer screenings
In a growing number of cases, the era of precision medicine has flipped the perceptions about rare and common cancers upside down.

Having cancer is challenging enough, but being diagnosed with a rare cancer often comes with additional difficulties. Many rare cancers are hard to identify, potentially resulting in missed or late diagnoses. It’s also difficult to find doctors with experience in treating these diseases, and less information is typically available about them. Because they affect far fewer patients, by definition, relatively few clinical trials devote time and resources to developing new treatments for rare cancers, which often means fewer options are available to treat them.

But in a growing number of cases, the era of precision medicine has flipped the perceptions about rare and common cancers upside down. Research has shown that some cancers long considered common are just a collection of rare subtypes that happen to share the same origin. Advances in immunotherapy have also led to new treatments for some rare cancers that once were difficult to treat. And advanced genomic testing has revealed molecular features in some rare cancers that may be treated with drugs previously developed for more common forms of the disease. “The definition of rare cancers has changed over time, both because of the role of genomic testing and evidence that different subtypes provide clinically meaningful information that impacts treatment and outcomes,” says Maurie Markman, MD, President of Medicine & Science for Cancer Treatment Centers of America® (CTCA).

Rare cancer facts

  • Rare cancers account for about 20 percent of all cancer diagnoses in adults.
  • In Europe, a rare cancer is defined as one in which there are 6 or fewer cases diagnosed per 100,000 people per year, less than half the number (15) used in the United States.
  • Most rare cancers are diagnosed in children, adolescents, and young adults. Rare cancers account for 71 percent of diagnoses in patients younger than 20, and 39 percent in patients 20 to 39 years old.

From adamantinoma (a rare form of bone cancer) to Waldenström's macroglobulinemia (a type of non-Hodgkin lymphoma), more than 500 forms of rare cancer have been documented, according to the National Cancer Institute (NCI). Rare cancers are not just those with unpronounceable names found in specific cells or obscure body parts. They also include cancer of the stomach, esophagus, bone, vagina, penis and gallbladder.

Select rare cancers  New cases 
 per 100,000 
Kaposi sarcoma
Abdominal (peritoneum
and retroperitoneum)

Because these cancers are so unusual, information on how to identify, stage or treat them is often difficult to find. “In settings where there is limited experience treating cancers, management of rare cancers is a particular challenge,” Dr. Markman says. “A physician may only see such a case every few years, or even less frequently. And the absence of clinical trials for rare cancers only makes the situation more difficult.”

While deepening their understanding of a specific cancer’s blueprint, scientists and oncologists have discovered that some so-called common cancers may not be that common at all. “Common cancers are being subclassified into clinically relevant, molecularly defined subgroups, and might lose their common cancer designation as a result,” the author of a 2016 article on rare cancers wrote in The Lancet Oncology, adding that “rare is the new common in the post-genomics era.” For instance, melanoma, is the fifth most common cancer diagnosed in the United States, with more than 90,000 new cases a year. But scientists have broken down melanoma into multiple subtypes, each with unique DNA mutations. Melanoma “is a collection of rare cancers, each with a unique biology with distinct implications for therapy—an increasingly common pattern in oncology,” the researchers wrote. Each of these subtypes would be considered rare cancers under NCI guidelines.

Most common cancers  New cases 
per  100,000  
Female breast cancer
Lung and Bronchus
Melanoma of the skin

Genomic analyses of a rare cancer may also unveil common mutations found in a variety of cancers, which may lead to new treatments. “We may be seeing treatments for rare cancers based on genomic analysis initially conducted on many common cancers or where the mutations are more commonly observed,” Dr. Markman says. “For example, a HER-2 mutation is highly relevant in breast cancer, but the presence of this abnormality may help to define an effective treatment approach in a cancer where anti-HER-2 therapy is not normally considered standard-of-care.” Researchers are studying the targeted therapy drug trastuzumab (Herceptin®), which is often prescribed to treat HER2-positive breast cancer, as a potential treatment for salivary duct cancer and uterine serous carcinomas, rare and aggressive cancers that may have HER2- positive mutations.

These new discoveries ushered in by the era of precision medicine have helped lead to new treatments and potential treatments for rare cancers. In 2017, immunotherapy drugs known as checkpoint inhibitors were approved to treat Merkel cell carcinoma, an extremely rare form of skin cancer, and Hodgkin lymphoma, a rare blood cancer. Immunotherapy is also being researched as a potential treatment for other rare cancers, such as thymic cancer and mesothelioma.

Challenge of rare cancers

Still, cancers considered rare continue to present challenges to doctors and patients. According to a 2017 study by the American Cancer Society:

  • 59 percent of rare cancers are diagnosed at advanced stages, compared with 45 percent of common cancers.
  • The five-year survival rate for men with rare cancers is 55 percent, compared with 75 percent for common cancers.
  • The five-year survival rate for women with rare cancers is 60 percent, compared with 74 percent for common cancers.

“The most significant finding was seeing that collectively rare cancers are still likely to be diagnosed at later stages and overall survival is poor,” study author Carol DeSantis, MPH, told CURE magazine. Facing such overwhelming percentages can be devastating to patients. But Stephen Lynch, MD, Intake and Primary Care Physician at our Phoenix hospital, encourages patients to take control of their diagnoses and learn as much as they can about their cancer. “Many times, whether it is a common cancer or a real oddity, a lot of patients really don’t understand it,” Dr. Lynch says. “They’ve been told they have breast cancer. Well, breast cancer is many different diseases. They’ve been told they have lung cancer. That’s many different diseases. So, first and foremost is helping the patient understand the foundation of their disease.”

As scientists and researchers delve deeper into the genomics of each cancer, they are finding that every cancer, like every patient, has unique characteristics that often require individualized treatments. Dr. Lynch says it’s important to educate patients on what makes their specific cancer tick, despite its classification as common or rare. “Digging into that diagnosis and helping patients learn about the unique molecular aspects of their tumor will help them better understand what they are dealing with and what questions to ask,” he says.